Until recently, conventional CDMA detection has been accomplished by using single-user detectors in which each user is detected separately via a matched filter (or correlator) receiver. Detection of each user is done without regard for the existence of other co-channel users.
Recently, multiuser detection has been developed for wireless communication. In multiuser detection, information (e.g., code, timing, channel) of multiple users is jointly used to better detect each individual user. Conventional minimum mean square error (MMSE) multiuser detection may offer significant performance improvements over conventional CDMA detection, but at the cost of increased receiver complexity. In conventional symbol level MMSE multiuser detection, a linear transformation that minimizes the mean square error may be applied to the outputs of the conventional detector for each user of a single rate system in order to decouple the co-channel interfering users. The computation of the standard MMSE linear transformation may involve a matrix inversion, where the order of the matrix is proportional to the number of users. As the order of the matrix increases, the computation of the inverse matrix becomes more difficult.
In a multirate system, a high-rate data user operating at M times the data rate of a low-rate user, appears to a conventional MMSE detector as M low-rate users, causing a significant increase in the order of the matrix and resulting computation complexity. In other words, an MMSE linear transformation may be applied every low-rate symbol interval to the conventional detector outputs of all symbols from all users that occur in the low-rate user symbol interval.
Like reference symbols in the various drawings indicate like elements.